MP Christian Wakeford says his defection to the Labour party was not a last minute decision and took "many months of soul-searching."
Mr Wakeford, who represents Bury South, crossed the Commons following revelations about lockdown parties in Downing Street.
He called on the Prime Minister to resign, and said the country needed a government that "upholds the highest standards of integrity and probity".
He said Mr Johnson "and the Conservative Party as a whole have shown themselves incapable of offering the leadership and government this country deserves".
Mr Wakeford said a number of factors contributed to his decision to leave the Conservative party.
He said: "There's been quite a few different issues throughout many months now, whether that's been the free school meals issues, the Patterson issue, Partygate and universal credit.
"The Conservatives aren't even talking about the majority of this and a lot of it comes down to respect - and being fair - and I just don't think that's coming from the Conservative Party."
Bury South is now represented by Labour - despite Mr Wakeford winning the seat for the Conservatives with a slim majority of 402 in 2019.
Currently, if an MP defects, a by-election is not necessary.
But, in September 2020, he presented and backed the Recall of MPs (Change of Party Affiliation) Bill, enabling constituents to recall their MP and call a by-election if they 'voluntarily change their political party affiliation'.
Young Labour, who have publicly opposed Mr Wakeford's defection, said in a Tweet: "The Labour Party must uphold Bury South members’ right to choose their own Labour candidate and constituents should be able to reassess their MP."
But speaking to ITV News', Mr Wakeford said he was "supporting a friend" when he co-sponsored that bill and does not think there should be a by-election.
He said: "I think I've not really changed, just the colour of a rosette I'm going to be wearing. I'm still a moderate."
Concerns have been brewing in his constituency of Bury South, with some voters saying they did not vote for a Labour MP.
Mr Wakeford, however, says he firmly remains a "centrist" and will continue to represent his constituents whoever they voted for.
In a message to his voters, the now Labour MP said: "I would say they elected a centrist and they still have a centrist.
"A centrist who actually wants to tackle the cost of living crisis and make sure they are all best represented and I genuinely think the best way to do that now is under Keir Starmer."
Mr Wakeford had been one of seven Tory MPs to publicly call for Mr Johnson to quit.
He was cheered by Labour MPs as he arrived in the Commons chamber for Prime Minister's Questions and sat behind Sir Keir Starmer.
The Labour leader began Prime Minister's Questions by "warmly welcoming" Mr Wakeford to his new party.
But the Prime Minister told Sir Keir: "The Conservative Party won Bury South for the first time in a generation under this Prime Minister on an agenda of uniting and levelling up and delivering for the people of Bury South. We will win again in Bury South."